Question

Yolk sac involutes 3-4 weeks after conception in

• A. Mare
• B. Dog
• C. Cat
• D. Pig

Hint:

Remember the unique transition in equine gestation:
Up to day 21: Choriovitelline (yolk sac) placenta is dominant.
Days 21-28 (3-4 weeks): Yolk sac undergoes rapid involution as the chorioallantoic placenta develops.

Answer 1

The Correct Option is A

Step 1: Understanding the Concept:
During early embryonic development in domestic mammals, the yolk sac (vitelline sac) forms a temporary vascular connection with the chorion, establishing the choriovitelline placenta (yolk sac placenta).
The lifespan and involution of the yolk sac vary considerably across species depending on the timing of chorioallantoic placentation.

Step 2: Detailed Explanation:
In the mare (horse), the yolk sac is initially very large and forms a highly functional choriovitelline placenta during early gestation.
This yolk sac placenta provides nutrients to the developing embryo during the first few weeks when the allantois is still small.
However, as the allantois expands and fuses with the chorion to form the permanent chorioallantoic placenta, the yolk sac rapidly regresses.
In the mare, this involution begins and is largely completed around 3 to 4 weeks (approximately day 21 to 28) after conception.
By day 28 of gestation, the chorioallantoic placenta takes over fully, and the yolk sac becomes a vestigial, small structure.
In contrast, in carnivores (dogs and cats), the yolk sac persists as a large, fluid-filled, vascularized structure until parturition.
In pigs, the yolk sac is transient but regresses even earlier, and they establish a diffuse chorioallantoic placenta very rapidly.

Step 3: Final Answer:
Therefore, the yolk sac involutes 3-4 weeks after conception in the mare, matching option (A).